Professor: Car-pool lanes don't always move more people

When we mentioned last month that a professor had concluded
car-pool lanes don't always ease congestion -- and often make it
worse -- many of you said, "Hey, wait a minute."

UC Berkeley professor Pravin Varaiya made the case in a fall
2005 study that car-pool lanes restrict flow to the point that some
freeways move fewer cars than they would if, say, all four, five or
six lanes in a particular direction were open to all vehicles.

But, as many of you noted, the number of cars isn't the key
statistic.

Rich Wilson of Santa Barbara put it this way: "Measuring cars
per hour seems to be a completely backwards way to measure the
efficiency of car-pool lanes. Since the idea of car-pool lanes is
to encourage people to car pool, I would think the important
measure would be people per hour."

Now, before we address the people-per-hour question, let's be
clear: Varaiya stated he was not talking about San Diego County's
two-lane car-pool system on Interstate 15, which is being extended
from Rancho Penasquitos to Escondido.

"I-15 is a different story," Varaiya said.

Because the I-15 system has more than one lane and is physically
separated from the mainline freeway by a concrete barrier, those
managed lanes move substantially more people per lane per hour than
do general-purpose lanes. Indeed, San Diego's car-pool-freeway
within a freeway is considered a model for how to squeeze the most
capacity out of a highway.

When Varaiya challenged car pooling, he was referring to the
typical system with one high-occupancy-vehicle lane separated only
by paint from other lanes. He argued that speeds slow in rush-hour
because faster drivers cannot pass slower ones, and because there
is always someone unwilling to drive 65 or 70 mph when cars are
crawling a few feet away in the next lane over.

In that single-lane scenario, slowing speeds mean the car-pool
lane is performing only marginally better than other lanes. Varaiya
said traffic counts suggest a typical diamond lane moves 100 or 200
more vehicles per hour than other lanes in rush-hour. If the
car-pool lane were converted into a general-purpose lane, all lanes
would perform better and the freeway would move more cars overall,
he maintained.

Now, let's get to the heart of the matter -- number of people
moved. Varaiya insists that, by that measure, too, freeways with
single-lane car-pool systems are not efficient.

"The data don't show you move more people with HOV," he
said.

Varaiya said that is because car pools, as a proportion of
overall commuter traffic, have declined sharply in Southern
California this decade (from 13.0 percent to 10.5 percent in San
Diego County, according to the U.S. Census Bureau), and because
studies show a majority of car pools are comprised of family
members.

"So they are not going to drive in two cars just because there
is no HOV lane," Varaiya said. "The issue really boils down to how
many car-poolers are still around when there is no HOV lane." He
maintained that, in the absence of one, most would continue to
share rides.